There is a shortage of organs, tissues or cells available for transplantation in recipients such as humans. Xenotransplantation or allotransplantation of organs, tissues, or cells into humans has the potential to fulfill this need and help hundreds of thousands of people every year. Non-human animals can be chosen as organ donors based on their anatomical and physiological similarities to humans. Additionally, xenotransplantation has implications not only in humans, but also in veterinary applications.
However, unmodified wild-type non-human animal tissues can be rejected by recipients, such as humans, by the immune system. Rejection is believed to be caused at least in part by antibodies binding to the tissues and cell-mediated immunity leading to graft loss. For example, pig grafts can be rejected by cellular mechanisms mediated by adaptive immune cells.